1. Field of the Invention
The present invention relates to a micro power cell with microfluidic-chip that can generate electrical energy by the streaming potential of fluid flowing through microchannels.
2. Description of the Prior Art
When fluid flows through a charged channel by pressure gradient, the density of counter-ions whose charge is opposite to that of the wall surface becomes higher near the surface of the channel wall, resulting in an establishment of the electric double layer (so-called Debye layer). The counter-ions in the electric double layer generate the streaming current by the flow to the direction of pressure gradient. Due to the potential difference causing by distribution gradient between the co-ions and the counter-ions, the streaming potential generates between upstream and downstream of the channel. Since the counter-ions are accumulated in the downstream, the counter-ions move to the opposite direction of the pressure gradient (i.e., the opposite direction of the fluids flow) by the back diffusion effect, and thus conduction currents are induced. Note that the summation of the streaming current and the conduction current is zero at steady state, meaning the conservation of net current in the channel.
When electrolyte solution prepared with arbitrary ionic concentration (i.e., the ionic strength) flows inside the straight cylindrical channel having a well-defined circular cross-section, the relationship between the zeta potential ζ and the streaming potential E is given by equation 1 referred to as Helmholtz-Smoluchowski (H-S) equation.
                                          Δ            ⁢                                                  ⁢            E                                Δ            ⁢                                                  ⁢            P                          =                                            ɛ              o                        ⁢                          ɛ              r                        ⁢            ζ                                η            ⁢                                                  ⁢                          λ              o                                                          [                  Equation          ⁢                                          ⁢          1                ]            
wherein
E denotes the potential induced by the electrokinetic effect, i.e., the streaming potential,
ΔP denotes the pressure difference exerted between both ends of the channel,
∈o denotes the dielectric constant at vacuum or vacuum permittivity,
∈r denotes the relative permittivity of the electrolyte solution,
λo denotes the electric conductivity of the electrolyte solution, and
η denotes the viscosity of the electrolyte solution.
For either high ionic concentration of electrolyte solution or low surface potential, the zeta potential obtained by measurements fairly agrees with that estimated by equation 1.
Many researchers have tried to understand the electrokinetic phenomena, and to analyze the fluid flowing in the microchannel with diameter less than several hundreds micrometers and the streaming potential according to it.
About 40 years ago, Rice and Whitehead addressed a correction factor from the analytic solution of Poisson-Boltzmann (P-B) equation that should be applied to the case of using the H-S equation (i.e., equation 1) in the paper [C. L. Rice, R. Whitehead, “Electrokinetic flow in a narrow cylindrical capillary”, J. Phys. Chem., 69, 4017-4024, 1965].
However, above research is limited to the 1:1 type electrolyte solution with low surface potential because the Debye-Hückel approximation is applied.
Levine et al. presented an analytic solution of the P-B equation that is applicable to monovalent symmetric electrolyte solution with same mobilities for full range of the surface potential in the paper [S. Levine, J. R. Marriott, G. Neale, N. Epstein, “Theory of electrokinetic flow in fine cylindrical capillaries at high zeta-potentials”, J. Colloid Interface Sci., 52, 136-149, 1975].
The streaming potential, which is an opposite mechanism to the electro-osmosis, is one of the electrokinetic phenomena. The streaming potential has been considered as a useful method for determining the unknown zeta potential, corresponding to the surface potential of charged material. From studies on the porous membrane filtration disclosed in Causserand et al.'s paper [C. Causserand, M. Nyström, P. Aimar, “Study of streaming potentials of clean and fouled ultrafiltration membranes”, J. Membr. Sci. 88, 211-222, 1994] and Szymczyk et al.'s paper [A. Szymczyk, B. Aoubiza, P. Fievet, J. Pagetti, “Electrokinetic phenomena in homogeneous cylindrical pores”, J. Colloid Interface Sci. 216, 285-296, 1999], it can be known that the measurement of electrokinetic streaming potential effectively contributes to the charge characterization of the pore and the surface of porous material.
As disclosed in Sung et al.'s paper [J. H. Sung, M.-S. Chun, H. J. Choi, “On the behavior of electrokinetic streaming potential during protein filtration with fully and partially retentive nanopores”, J. Colloid Interface Sci. 264, 195-202, 2003], or Chun et al.'s U.S. Patent [M.-S. Chun, J.-J. Kim, S.-Y. Lee, “Equipment and method of local streaming potential measurement for monitoring the process of membrane fouling in hollow-fiber membrane filtration”, U.S. Pat. No. 6,277,099 B2, 2004], important information about the colloidal particle deposition onto the surface of the porous membrane can be obtained by monitoring the dynamic behavior of electrokinetic streaming potentials with time progress.
The physical meaning of above equation 1 is that when the electrolyte solution flows in a charged channel with a pressure gradient ΔP, the potential difference ΔE occurs between the ends of the channel. Accordingly, connecting the external circuit applied appropriate resistance provides the electrical energy encountered by electric current and potential. Recently, a research on the possibility of the streaming potential generated by the electrokinetic principle to the energy source has been presented. In the Daniel Kwok's paper [J. Yang, F. L. Larry, W. Kostiuk, D. Y. Kwok, “Electrokinetic microchannel battery by means of electrokinetic and microfluidic phenomena”, J. Micromech. Microeng., 13, 963-970, 2003], when the tap water flows by the pressure difference in a microporous glass filter (diameter 2 cm, thickness 3 mm, Schott-Duran, Mainz) structured with disordered pores with pore size of 10˜16 μm and maximum porosity of 60%, it was reported that the maximum value of the streaming potential of 10V and the maximum current of a few μA could be obtained.
However, a power cell relevant to streaming potential that could apply to the practical operation has not been developed until now.
Both the MEMS process and micromachining technologies allow us to fabricate microchannels with desired channel dimension. Using the lab-on-a-chip technique based on these technologies, the micro total analysis system (μ-TAS) as well as the high throughput system (HTS) can be realized. As shown in the research of Shoji and Esashi [S. Shoji, M. Esashi, “Microflow devices and systems” J. Micromech. Microeng., 4, 157-171, 1994], the early researchers from 1980s to the middle of 1990s mainly developed the microfluidic devices by means of the silicon-based micromachining.
As the instruments in the semiconductor industry have advanced, the paradigm of the fabrication technology regarding microfluidic devices has also been changed. After the middle of 1990s, micromachining technology for the disposable plastic materials that are easily replicable in mass has been developed. The master mold for replication can be made by the traditional silicon-based micromachining technology. Like the study of Jo et al. [B.-H. Jo, L. M. van Lerberghe, K. M. Motsegood, D. J. Beebe, “Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer”, J. Microelectromech. Sys., 9(1), 76-81, 2000], a method using photoresist SU8 and polydimethylsiloxane (PDMS) is widely known as a simple and low-cost technology.
Since PDMS is indeed inert and transparent in the range of visible or near UV lights, it has merits as a material for microfluidic device. To make the microchannel, a master mold conforming to the microchannel shape is formed on the silicon wafer. Liquid PDMS prepolymer is poured over the mold, and then cure it. Subsequently, the PDMS replica is peeled from the master, and the replica is sealed to a glass coverslip to enclose the channels.
As disclosed in the study of McDonald and Whitesides [J. C. McDonald, G. M. Whitesides, “Poly(dimethylsiloxane) as a material for fabricating microfluidic devices”, Acc. Chem. Res., 35(7), 491-498, 2002], the above method is easier in process and lower in cost than the conventional method of etching the glass or silicon wafer in view of mass production of microfluidic-chip.